It is very common nowadays to come across businesses to organise events as part of their marketing strategy. They will also invite other businesses to join in and become sponsors for this events.
If the organiser has to provide identifiable benefits such as advertising or publicity to the sponsors in return for the sponsorship payments, GST has to be charged and collected from the sponsors. This is because the identifiable benefits are considered a supply by the organiser to the sponsors in return for a consideration (the sponsorship payments).
On the other hand, if the sponsorship payments does not involve any identifiable benefits in return (which is very rare!!), then there is no consideration for a supply and GST need not be accounted for.